1. Field of the Invention
The present invention relates to a transfer device for transferring an image on an image carrier to a recording material. More particularly, the present invention relates to improvements in a transfer device comprising a transferring member for transferring a recording material through a nip between the image carrier and the transferring member, an image-forming apparatus using the transferring member and method for producing the transferring member.
2. Description of the Related Art
By way of example, a conventional image-forming apparatus has been already provided in a form which operates in an electrophotographic process comprising the steps of forming an electrostatic latent image on an image carrier (which widely includes a latent image carrier such as photoreceptor drum, a combination of latent image carrier and intermediate transfer drum for intermediately transferring and retaining an image on the latent image carrier), developing the electrostatic latent image with a predetermined toner by a developing device, and then transferring the toner image formed on the image carrier onto a recording material via a transfer device.
As such a transfer device there has been known a non-contact type transfer device such as corotoron. This non-contact type transfer device is disadvantageous in that it causes troubles with generation of ozone. The recent trend is for more so-called contact type transfer device to be used which transfers a toner image on the image carrier to a recording material in a contact process while conveying the recording material through a nip between a transfer roll disposed in contact with or in the vicinity of the image carrier and the image carrier.
For this contact type transfer device, there has been often used a transfer roll comprising a metallic roll coated with a fluorinated rubber layer.
In order to effectively prevent troubles such as attachment of residual toners to this type of transfer roll, a cleaning device comprising a cleaning blade disposed in contact with the transfer roll is provided.
Such a cleaning blade may be made of an elastic material such as urethane rubber to prevent damage on the fluorinated layer which is a surface coat layer on the transfer roll.
In this type of transfer device, the frictional resistance of the cleaning blade with the surface of the transfer roll can be suppressed to a relatively small value due to the surface treatment of the transfer roll. However, this type transfer device is technically disadvantageous in that as the recording material runs, more external additives for toner are attached to the surface of the transfer roll, raising the coefficient of friction of the cleaning blade with the surface of the transfer roll and hence giving more rotary load to the transfer roll. Thus, the surface of the transfer roll cannot be cleaned at a low torque.
As a result, a high rotary torque is needed to rotate the transfer roll in a stable manner. This accordingly raises the cost of driving source to disadvantage.
In order to accurately control the density of an image transferred onto the recording material, a density control process which controls image density has been already proposed, e.g., by forming a density patch for density control on an image carrier, transferring the density patch to the surface of a transfer roll, and then detecting the density whereby the density corresponding to an image transferred to the recording material can be directly detected (see JP-A-7-168401 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)).
However, the foregoing transfer device has a transfer roll comprising a surface rubber layer coated with a fluorinated layer and thus can hardly give specular reflection as a surface optical property. For example, even if a density patch is formed on the surface of the transfer roll, the density of the density patch can hardly be optically detected.
This density control process is also technically disadvantageous in that even if the density patch transferred to the transfer roll can be removed by the cleaning blade, residual toner gradually stains the surface of the transfer roll, the reflectance of the surface of the transfer roll is lowered, and hence the detection of density patch is inaccurate performed.
In particular, when the toner used comprises substantially spherical particles, it is more likely that the toner can pass through the cleaning blade to make the foregoing technical problems remarkable.
As a method for cleaning the surface of a hard and smooth transfer roll, there has been proposed that a metallic scraper is effective (as in JP-A-6-324583). However, there is no further specific disclosure relating to the transfer roll.
On the other hand, as a prior art of a transfer roll, there has been proposed a transfer roll comprising a first layer made of an elastic material and a second layer made of a resin having a higher resistance than that of the first layer (as in JP-A-3-202885). As the second layer (a surface layer), there has been disclosed one comprising a polycarbonate, polyester, nylon or the like as a base.
There is an apprehension that when the foregoing metallic scraper is applied to the transfer roll having such a constitution, the surface layer of the transfer roll is scratched or abraded in a short period of time to cause imperfect cleaning or defective detection of density patch.
In order to solve these technical problems, the present applicant proposed a transfer device provided with a transfer member (e.g., transfer roll) having a polyimide resin layer formed on the surface thereof (Japanese Patent Application No. 2000-278014).
In accordance with this type of transfer device, the constitution of a hard transfer roll having a metallic roll with a polyimide resin layer provided thereon makes it possible to eliminate the apprehension of scratching or abrasion even if the metallic scraper comes in contact with the transfer roll because the polyimide resin layer has a high mechanical strength.
In this transfer roll having a polyimide resin layer formed on the surface thereof, the polyimide resin layer is provided with some electrical conductivity to assure desired transferring properties. However, in view of manufacturing cost, the polyimide resin layer is thin. Thus, the resistance of the polyimide resin layer must be set to a somewhat high value to prevent the leakage of current between the metallic roll and the image carrier.
As a result, charge can be easily accumulated in the polyimide resin layer. Thus, there is an apprehension that when transfer is conducted in a high electric field as in printing on OHP sheet or cardboard or double-sided printing, a sufficient transfer electric field cannot be obtained, causing imperfect transfer.
Even if an epoxy resin layer having a high abrasion resistance is provided on the surface of the metallic roll instead of polyimide resin layer, there is an apprehension that the accumulation of charge causes imperfect transfer as in the case of polyimide resin layer because the epoxy resin layer has an extremely high resistance, although the cleaning properties of the metallic scraper may be kept good.
The present invention has been worked out to solve the foregoing technical problems. An object of the present invention is to provide a transfer device which can be cleaned at a low torque while maintaining good transferring properties and, when a method is employed which forms a process control image such as a density control patch on a transferring member, the transfer device can certainly accomplish the detection of process image, an image-forming apparatus using the transfer device, and method for producing the transferring member.
In other words, as shown in FIGS. 1A and 1B, according to the present invention, there is provided a transfer device for transferring an image on an image carrier 1 to a recording material 2, the transfer device comprising a transferring member 4 adapted to nip and convey the recording material 2 between the transferring member 4 and the image carrier 1, a guard resin layer 5 having a surface microhardness of not smaller than 18 as measured under a test load of 2.0 gf (19.6 mN) and a load rate of 0.0145 gf (0.1421 mN)/sec by a Type DUH-201S dynamic ultramicrohardness meter produced by Shimadzu Corp. with a triangular pyramid indenter having 115xc2x0 in a ridge angle, the guard resin layer provided on a surface of the transferring member, and an adjustment resistance layer 6 provided as a ground layer of the guard resin layer 5, the adjustment resistance layer 6 adapted to inhibit an accumulation of charge in the guard resin layer.
In this technical means, the image carrier 1 widely includes an image-forming carrier such as latent image carrier and an intermediate transfer material for intermediately retaining an image from this image-forming carrier so long as these carry an image.
The transferring member 4 is not limited to a roll but may be in a form of a belt so long as the transferring member 4 nips and conveys the recording material 2 between the transferring member 4 and the image carrier 1.
Furthermore, the transferring member 4 comprise a guard resin layer 5 on a surface thereof and an adjustment resistance layer 6 as a ground layer of the guard resin layer 5. For example, when the transferring member 4 is in the form of the roll, the transferring member 4 may often comprise a metallic core 7 for securing enough rigidity for nipping and conveying between the image carrier 1 and the transferring member 4 and a guard resin layer 5 provided on periphery of the core 7 with an adjustment resistance layer 6 interposed therebetween.
There is an apprehensiveness that the transferring member 4 may be subject to attachment of image-forming particles such as toner or external additives to the surface thereof. In order to clean the surface of the transferring member 4, a cleaning scraper 8 is normally provided so as to contact with the guard resin layer 5 on the transferring member 4.
The guard resin layer 5 has surface microhardness of not smaller than surface microhardness corresponding to a polyimide resin.
The term xe2x80x9csurface microhardnessxe2x80x9d as used herein is meant to indicate microhardness of a surface portion of the guard resin layer 5 rather than total hardness of the guard resin layer 5 and the adjustment resistance layer 6. Paying attention to the fact that grinding has an effect on the microhardness of the surface portion of the guard resin layer, the polyimide resin which has the highest microhardness as affiars stand is took for a comparative standard, and a material having a microhardness of not smaller than that of polyimide resin is considered practically acceptable.
Measurement of the microhardness can be accomplished by method defined in JIS. Alternatively, other methods independently determined with existing surface microhardness meters may be properly employed. Accordingly, any surface microhardness meter can be used so long as the guard resin layer has a surface microhardness not smaller than that of polyimide resin regardless.
Generall, measurement principle of the surface mircrohardness is shown in FIG. 1B. A needle penetrator 9 having a predetermined shape is pressed against the surface of the guard resin layer 5 to a predetermined load P (mN). Supposing that the penetration depth of the penetrator 9 is D (xcexcm), the surface microhardness of the guard resin layer is the greater, D is the smaller. The surface microhardness DH is represented by, e.g., the following equation:
DH=xcex1xc2x7P/D
where xcex1 is a coefficient predetermined by shape of the penetrator 9 and measurement conditions.
An example is shown in which surface microhardness is predetermined by specific hardness meter. The surface microhardness of the guard resin layer 5 is not smaller than 18 as measured under a test load of 2.0 gf (19.6 mN) and a load rate of 0.0145 gf (0.1421 mN)/sec by a Type DUH-201S dynamic ultramicrohardness meter produced by Shimadzu Corp. with a triangular pyramid penetrator having a ridge angle of 115xc2x0.
The term xe2x80x9csurface microhardness of not smaller than 18xe2x80x9d as used herein is meant to indicate that since the surface microhardness of polyimide resin is in a range of 18 to 50 as measured under the same conditions as mentioned above, the lower limit is used.
The guard resin layer 5 preferably has contact angle of not smaller than 70xc2x0 with respect to water.
The contact angle with respect to water is determined by surface energy and surface shape (roughness) of the material. When the guard resin layer 5 has a contact angle of not smaller than 70xc2x0 with respect to water, the guard resin layer 5 is hardly attract image-forming particles and external additives and is easily cleaned with the scraper 8 to advantage.
In general, a polyimide resin has an initial contact angle in a range of 70xc2x0 to 80xc2x0 and shows a contact angle drop in a range of about 5xc2x0 to 10xc2x0 after abrasion.
Thickness of the guard resin layer 5 is properly predetermined, and is normally in a range of 10 xcexcm to 100 xcexcm.
When the thickness of the guard resin layer 5 falls below 10 xcexcm, the guard resin layer 5 is subject to problem on strength during producing process and cleaning process. On the contrary, when the thickness of the guard resin layer 5 exceeds 100 xcexcm, the guard resin layer 5 is disadvantageous in producibility, cost and transferring properties.
It is preferred that the guard resin layer 5 is hardly deformed when brought into contact with the scraper 8. Thus, the guard resin layer 5 preferably has a Young""s modulus of not smaller than 200 kg/mm2.
When the Young""s modulus of the guard resin layer 5 is too small, since an outer diameter thereof is changed or unevenness of the adjustment resistance layer 6 is appeared on the surface of the guard resin layer 5, the cleaning properties of the scraper 8 is impaired.
In general, a polyimide resin has a Young""s modulus of 200 kg/mm2 at minimum and normally not smaller than 400 kg/mm2.
In order to keep transferring properties of the transferring member 5 more fairly, the guard resin layer 5 preferably has an electrically-conductive material (e.g., resistance-adjusting material such as carbon) dispersed therein.
This is because the dispersion of the electrically-conductive material makes it possible to easily accomplish the adjustment of resistance of the guard resin layer 5.
As the electrically-conductive material to be dispersed in the guard resin layer 5, there may be properly selected from the group consisting of electronically-conductive material such as carbon black and metal oxide and ionically-conductive material such as quaternary ammonium salt. In practice, however, the electronically-conductive material is preferred because it has little environmental dependence.
In order to further enhance the resistance retention or uniformity of the guard resin layer 5, it is preferred that an electrically-conductive polymer material be used as an electrically-conductive material.
With respect to the surface properties of the transferring member 4, that is, the surface properties of the guard resin layer 5, in order to maintain the cleaning properties by the scraper 8, it is preferred that surface roughness of the transferring member 4 is not greater than the minimum diameter of the image-forming particles.
According to this arrangement, a phenomenon can be avoided that the image-forming particles are caught by indentation on the surface of the transferring member 4.
The adjustment resistance layer 6 may be properly selected so far as the adjustment resistance layer 6 can inhibit the accumulation of charge on the guard resin layer 5 to keep the transferring properties good. In practice, however, the adjustment resistance layer 6 preferably has elasticity so that a nip region having a predetermined width is formed between the transferring member 4 and the image carrier 1.
In accordance with this embodiment, a wide nip region can be secured without raising nip pressure between the transferring member 4 and the image carrier 1.
In relation to preferred embodiment of elasticity, the adjustment resistance layer 6 preferably has an Asker C hardness of not smaller than 20.
Since a sufficient tension is obtained between the guard resin layer 5 and the adjustment resistance layer 6, it is preferred to use a tubular polyimide resin as mentioned later as the guard resin layer 5.
In a preferred embodiment of the adjustment resistance layer 6 for preventing the accumulation of charge on the guard resin layer 5, the adjustment resistance layer 6 has a resistance in a range of 106xcexa9 to 109xcexa9 when 1,000 V is applied thereto and the guard resin layer 5 has a lower resistance than resistance of the adjustment resistance layer 6.
With regard to relationship between the guard resin layer 5 and the adjustment resistance layer 6, in view of keeping the cleaning properties good, it is preferred that the modulus of the guard resin layer 5 is greater than that of the adjustment resistance layer 6.
When the modulus of the guard resin layer 5 is not greater than that of the adjustment resistance layer 6, unevenness on the adjustment resistance layer 6, which is a ground layer, appears on the surface of the tubular guard resin layer 5 so that there is an apprehension to adversely affect the cleaning properties. On the contrary, when the modulus of the guard resin layer 5 is greater than that of the adjustment resistance layer 6, the adverse effect on the cleaning properties can be effectively avoided.
The constitution of the transferring member 4 is accomplished by any known method.
For example, the guard resin layer 5 may be formed by any known coating method such as flow coating and dipping. Alternatively, a tube or the like may be used as the guard resin layer 5. No matter whatever method is used, it is preferred that uniform flatness is secured.
A typical embodiment of the transferring member 4 is a transferring member provided with a tubular guard resin layer 5.
The foregoing embodiment of the transferring member 4 is prepared by a process which comprises the steps of preparing an inner structure having an adjustment resistance layer 6 provided on periphery of a base member such as core 7, and inserting the inner structure into a tube serving as a guard resin layer 5.
Then, in order to accomplish preferred state between the produced tubular guard resin layer 5 and the inner structure, it is necessary that the tube serving as the guard resin layer 5 closely adheres to periphery of the inner structure.
In this producing method, there are a method of assisting the insertion by air and else to easily realize the insertion process into the tube serving as the guard resin layer 5. For example, after cooling the inner structure at a low temperature, the inner structure is inserted into the tube serving as the guard resin layer 5.
In order to keep a adhesion between the produced inner structure and the tube serving as the guard resin layer 5 good, it is necessary that the inner structure is expanded under preferred condition during the insertion process into the tube serving as the guard resin layer 5.
As the expansion conditions, the inner structure comprises the adjustment resistance layer 6 having a linear expansion coefficient so that an outer diameter of the inner structure at a time when the inner structure is cooled is smaller than an inner diameter of the tube serving as the guard resin layer at normal temperature and the outer diameter of the inner structure at normal temperature is greater than the inner diameter of the tube at normal temperature.
The material of the scraper 8 is not limited to metal. The material of the scraper includes a high hardness resin which can clean at low torque. However, the scraper 8 is preferably made of metal in view of cost.
The metal constituting the metallic scraper 8 is properly selected from SUS, phosphor bronze, and the like.
In this embodiment, the metallic scraper 8 comes in linear contact with the surface of the transferring member 4. Thus, the frictional resistance of the metallic scraper 8 with the surface of the transferring member 4 can be suppressed to an extremely small value to enable to clean the surface of the transferring member 4 at low torque.
With regard to the method for producing the metallic scraper 8, etching is preferable because the etching generates no burr on an edge of the product.
In order to further reduce the load of the metallic scraper 8 on the transferring member 4, the metallic scraper 8 is preferably coated with a low friction coat layer at least on the surface thereof contacting with the transferring member 4.
In order to prevent the metallic scraper 8 and the transferring member 4 from being caught by each other, the metallic scraper 8 is preferably formed to curve at an end in a longitudinal direction of the metallic scraper 8, the end contacts with the transferring member 4.
In using of the metallic scraper 8, it is necessary that leakage of transfer current through the metallic scraper 8 is prevented.
In this case, the metallic scraper 8 is supported so as not to connect to the ground.
The term xe2x80x9cbeing supported so as not to connect to the groundxe2x80x9d as used herein is meant to indicate that the metallic scraper 8 is supported and insulated from the ground or supported under application of the same voltage as that applied to the transferring member 4. In this arrangement, imperfect transfer due to leakage of transfer current is prevented.
In another embodiment the present invention, as shown in FIG. 1A, there may be provided a transfer device for transferring an image on an image carrier 1 to a recording material 2, comprising a transferring member 4 adapted to nip and convey the recording material 2 between the transferring member 4 and the image carrier 1, a guard resin layer 5 made of an epoxy resin, provided on a surface of the transferring member 4, and an adjustment resistance layer 6 provided as a ground layer of the guard resin layer 5, the adjustment resistance layer 6 having a smooth interface with the guard resin layer 5, the adjustment resistance layer 6 adapted to inhibit accumulation of charge in the guard resin layer 5.
In this embodiment, the requirement for xe2x80x9csmooth interface with the guard resin layer 5xe2x80x9d is based on the fact that if the adjustment resistance layer 6 has a rough surface, the surface of the guard resin layer 5 made of, e.g., epoxy resin cannot be rendered smooth to affect the transfer properties.
Of course, the embodiment of the transferring member 4 (including provision of the guard resin layer 5 with electrical conductivity and surface roughness of the transferring member 4) and the cleaning scraper 8 to be disposed in contact with the transferring member 4 can be properly selected as mentioned above.
In order to reduce frictional force of the guard resin layer 5 with, e.g., the scraper 8 in this technical means, the guard resin layer 5 made of an epoxy resin includes a fluororesin incorporated therein.
In order to effectively prevent the scraper 8 from being caught by indentations formed on an area of the transferring member 4 in contact with the scraper 8, the adjustment resistance layer 6 preferably has an Asker C hardness of not smaller than 70.
In an embodiment using the guard resin layer 5 made of the epoxy resin, in order to act the adjustment resistance layer 6, it is necessary that the adjustment resistance layer 6 is formed by a material having a low resistance than resistance of the guard resin layer 5 made of the epoxy resin.
Thus, the guard resin layer 5 has the surface microhardness of not smaller than the surface microhardness corresponding to the polyimide resin or is formed of the epoxy resin having the high abrasion resistance to enable to use the metallic scraper 8 or the like as a cleaning element and accomplish cleaning at low torque.
Further, the guard resin layer 5 which is the surface of the transferring member 4 is formed by, e.g., the polyimide resin or the epoxy resin, to render the surface layer of the transferring member 4 smooth and highly reflective.
The smoothness or reflectivity of the surface layer is normally determined during producing process of the polyimide resin or the epoxy resin. Of course, any proper post-treatment such as polishing may be conducted.
In such transfer device, a density patch or the like for density control can be formed on the transferring member 4 whereby information such as image density can be detected.
The present invention can be applied not only to the foregoing transfer device but also to an image-forming apparatus comprising this transfer device.
In this case, as shown in FIG. 1A, there is provided an image-forming device comprising an image carrier 1 adapted to carry an image, and a transfer device 3 adapted to transfer the image on the image carrier 1 to a recording material 2, wherein the transfer device 3 comprises a transferring member 4 adapted to nip and convey the recording material 2 between the transferring member 4 and the image carrier 1, a guard resin layer 5 having surface microhardness not smaller than surface microhardness corresponding to polyimide, the guard resin layer 5 provided on a surface of the transferring member 4, and an adjustment resistance layer 6 provided as a ground layer of the guard resin layer 5, the adjustment resistance layer 6 adapted to inhibit an accumulation of charge in the guard resin layer 5, or wherein the transfer device 3 comprises the guard resin layer 5 made of an epoxy resin, provided on a surface of the transferring member 4, and the adjustment resistance layer 6 provided as a ground layer of the guard resin layer 5, the adjustment resistance layer 6 having a smooth interface with the guard resin layer 5, the adjustment resistance layer 6 adapted to inhibit accumulation of charge in the guard resin layer. Furthermore, in addition to the above described, the guard resin layer 6 on the transferring member 4 is provided with a scraper 8 for cleaning to contact with the guard resin layer 6.
In order to realize production of an image with high quality, this image-forming apparatus may further comprises a process controlling unit adapted to control the image to be formed by forming a process control image (e.g., density patch for density control) on the transferring member 4 and detecting information of the process control image.
From another standpoint of view, in order to realize production of an image with high quality, it is preferred that spherical particles having shape coefficient of not greater than 130 is used as image-forming particles to be formed on the image carrier 1 to assure a high transferability.